For water treatment reverse osmosis (RO) is typically used. In RO, an applied pressure exceeds the osmotic pressure of an aqueous feed solution to produce substantially clean water, e.g. desalinated water. Forward osmosis (FO) differs from reverse osmosis (RO) in that no or little hydraulic pressure is applied. This has the advantage that little energy is needed for the process to take place. Potentially, forward osmosis is thus superior to reverse osmosis with respect to energy consumption, but the FO technology is not yet fully developed for various applications.
Osmosis can in generally be classified through the equation:Jw=A(σΔπ−ΔP)where Jw is the water flux or membrane flux, A the water permeability constant of the membrane, σ the reflection coefficient, Δπ the osmotic pressure differential across the membrane, and ΔP the applied pressure. Three regimes may be identified for a given membrane: ΔP=0; this is Forward osmosis. ΔP>Δπ; this is reverse osmosis, and, finally, ΔP<Δπ; which is so-called pressure-retarded osmosis (PRO). PRO is currently an experimental technique for power generation. For a recent review of forward osmosis, the skilled reader is referred to Cath et al., Forward Osmosis: Principles, applications, and recent developments, Journal of Membrane Science, 281 (2006) 70-87.
In osmotic processes, both RO and FO, the effective osmotic pressure difference is significantly lower than what would be expected from the bulk osmotic pressure difference. This reduces in turn the water flux through the membrane and therefore the yield of the process. This reduction is attributed to transport phenomena related to the membrane, particularly concentration polarization (CP). CP arises due to the local concentration build-up next to which is unfortunately rather slowly removed by diffusion, and the resulting boundary layer results in a much lower effective osmotic pressure difference. This is known as external CP. The boundary layer within the membrane is known as the internal concentration polarization (CP), and it can be reduced by optimised membrane design, whereas the external concentration polarisation can be reduced by cross-flow and turbulence at the membrane surface. Turbulence promoters are known to decrease the external boundary layers, but they suffer from increased energy consumption and pressure drops due to the friction from the promoters. It is also difficult to clean an osmosis system if many turbulence promoters, e.g. spacers and spiral wound channels, are present.
Hence, an improved forward osmosis device would be advantageous, and in particular a more efficient and/or reliable device would be advantageous.